In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio-access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into areas or cell areas, with each area or cell area being served by radio-network node such as an access node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. The area or cell area is a geographical area where radio coverage is provided by the access node. The access node operates on radio frequencies to communicate over an air interface with the wireless devices within range of the access node. The access node communicates over a downlink (DL) to the wireless device and the wireless device communicates over an uplink (UL) to the access node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio-access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High-Speed Packet Access (HSPA) for communication with user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for present and future generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several access nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio-network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural access nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, such as 4G and 5G networks. The EPS comprises the Evolved Universal Terrestrial Radio-Access Network (E-UTRAN), also known as the Long-Term Evolution (LTE) radio-access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a 3GPP radio-access technology wherein the access nodes are directly connected to the EPC core network. As such, the Radio-Access Network (RAN) of an EPS has an essentially “flat” architecture comprising access nodes connected directly to one or more core networks.
With the emerging 5G technologies such as New Radio (NR), the use of very many transmit- and receive-antenna elements is of great interest as it makes it possible to utilize transmit- and receive beamforming. Transmit-side beamforming means that the transmitter can amplify the transmitted signals in a selected direction or directions, while suppressing the transmitted signals in other directions. Similarly, on the receive side, a receiver can amplify signals from a selected direction or directions, while suppressing unwanted signals from other directions.
Beamforming allows the signal to be stronger for an individual connection. On the transmit side this may be achieved by a concentration of the transmitted power in the desired direction(s), and on the receive side this may be achieved by an increased receiver sensitivity in the desired direction(s). This beamforming enhances throughput and coverage of the connection. It also allows reducing the interference from unwanted signals, thereby enabling several simultaneous transmissions over multiple individual connections using the same resources in the time-frequency grid, so-called multi-user Multiple Input Multiple Output (MIMO).
A problem with beamforming is to decide which beam(s) to use for transmission and/or reception. To support Transmit (Tx) side beamforming at a radio-network node, a number of reference signals may be transmitted from the radio-network node, whereby the wireless device can measure signal strength or quality of these reference signals and report the measurement results to the radio-network node. The radio-network node may then use these measurements to decide which beam(s) to use for one or more wireless devices.
A combination of periodic and scheduled reference signals may be used for this purpose.
The periodic reference signals, typically called beam reference signals (BRS), are transmitted repeatedly, in time, in a large number of different directions using as many Tx-beams as deemed necessary to cover an operational area of the radio-network node. As the naming indicates, each BRS represents a unique Tx-beam from that radio-network node. This allows a wireless device to measure the BRSs when received in different beams, without any special arrangement for that wireless device from the radio-network node perspective. The wireless device reports information e.g. the received powers for different BRSs, or equivalently different Tx-beams, back to the radio-network node. The radio-network node may then transmit dedicated signals to that wireless device using one or more beams that are reported as strong for that wireless device. Because the BRSs are transmitted repeatedly, with a repetition period, over a large number of beams, the repetition period has to be relatively long to avoid using too many resources per time unit for the BRSs.
The scheduled reference signals, called channel-state information reference signals (CSI-RS), are transmitted only when needed for a particular connection. The decision when and how to transmit the CSI-RS is made by the radio-network node and the decision is signalled to the involved wireless devices using a so-called measurement grant. When the wireless device receives a measurement grant it measures on a corresponding CSI-RS. The radio-network node may choose to transmit CSI-RSs to a wireless device only using beam(s) that are known to be strong for that wireless device, to allow the wireless device to report more detailed information about those beams.
Alternatively, the radio-network node may choose to transmit CSI-RSs also using beam(s) that are not known to be strong for that wireless device, for instance to enable fast detection of new beam(s) in case the wireless device is moving.
The radio-network nodes of a NR network transmit other reference signals as well. For instance, the radio-network nodes may transmit so-called demodulation reference signals (DMRS) when transmitting control information or data to a wireless device. Such transmissions are typically made using beam(s) that are known to be strong for that wireless device.
As stated above, beamforming is not restricted to the radio-network node. It can also be implemented as Receive (Rx)-side beamforming in the wireless device, further enhancing the received signal and suppressing interfering signals. Care must then be taken to compare different Tx-beams only when the BRS is known to be received using the same (or similar) receive beams, otherwise the difference in received power may depend on the used receive beam rather than on the transmit beam.
When the wireless device is connected to a wireless communication network, the wireless device tries to maintain a time synchronization with the wireless communication network. This is facilitated by the radio-network node, which periodically transmits so-called synchronization signals. In LTE, these synchronization signals are defined by the standard and the wireless device constantly monitors these synchronization signals and adjusts its synchronization based on this monitoring. Typically, the monitoring consists of having a signal correlator searching for the synchronization signals directly in the time-domain. A similar solution is envisioned for the NR standard.
For the wireless device to be in synch means that the wireless device knows when subframe boundaries and/or Orthogonal Frequency-Division Multiplexing (OFDM) symbol boundaries will occur, and hence, the wireless device can adjust a subsequent signal processing to these boundaries. The first, and probably most important step, is to adjust a precise positioning in time of a Fast Fourier Transform (FFT)-window when transforming the received signal to the frequency-domain. If a wireless device moves such that a propagation delay from the radio-network node to the wireless device changes, the positioning of the FFT-window must change and it is the synchronization signals that provide the means to achieve this.
As described above, a wireless device must monitor the BRS transmitted from the radio-network node in order to report back the BRS received power (BRS-RP) so that the radio-network node can decide which Tx-beams to use for data transmission. The BRSs are unique to each radio-network node, at least locally within a reasonably large geographical area, so that a wireless device can measure and report BRS-RP on beams from a neighboring transmission point without ambiguity. This is necessary in order to support mobility between radio-network nodes.
A problem with existing solutions is that in order to measure accurately on a BRS the wireless device must be in synch with the radio-network node transmitting the BRS. However, in order to support mobility of the wireless device between radio-network nodes, it must be possible for the wireless device to measure on BRSs from more than one radio-network node, and be in sync with each of these radio-network nodes. If this is not achieved the communication will fail resulting in a limited or reduced performance of the wireless communication network.